1. Field of the Invention
The present invention relates to a system for controlling ignition timing in an engine, wherein feedback control is effected such that ignition timing is delayed when knocking is detected and timing is advanced when no knocking is detected.
2. Description of the Prior Art
Knocking in an engine is a dangerous phenomenon which may cause melting loss to pistons and exhaust valves and which leads to destruction of the engine in extreme cases. In the conventional systems for controlling ignition timing in engines, to avoid the occurrence of serious trouble due to knocking, ignition timing is retarded from the ignition timing where knocking may occur by a predetermined amount.
Description will hereunder be given to the relationship between the probability of knocking and the ignition timing control level with reference to the graph of the knocking cycle percentage as shown in FIG. 1. In FIG. 1, the percentage of engine cycles in which knocking occurs is given as an abscissa and the ignition timing as an ordinate. In the drawing, if the ignition timing is advanced, a strong knocking condition takes place, with the result that, for example, at the point C, the percentage of engine cycles in which knocking is detected is increased as at the point A. It is possible to reduce the knocking cycle percentage from the point A. to the point B by retarding the spark from the point C to the Point D.
However, there have been the following disadvantages with the conventional ignition system for controlling the frequency of the occurrence of knocking.
(1) A detector for detecting the occurrence of knocking has its limit in sensitivity, with the result that, even in the case the knocking condition occurs in reality, if the condition does not reach the extent which can be detected by the detector, it has been impossible to control the frequency of knocking. PA0 (2) There have been such cases that the detectors for detecting the occurrence of knocking can sensitively detect the occurrence of knocking in cylinders adjacent to the mounting positions of the detectors, but the detectors cannot detect the occurrence of knocking in cylinders remote from the mounting positions of the detectors because of low sensitivity of the detectors. Consequently, in the case it is designed that the ignition timings of all the cylinders are made equal, if the knocking condition tends to easily occur in a specific cylinder due to a lack of fuel distribution, different conditions of deposits in combustion chambers or the like, the detector may not detect the knocking condition in said specific cylinder which would be located remote from the detector, with the result that such cases have occurred where the control of ignition timing is not effected even if the frequency of the occurrence of knocking becomes high.